1. Field of the Invention
The present invention generally relates to a process of electrodepositing platinum and platinum-based alloy nano-particles using an electrodeposition solution containing ethylene glycol.
2. Description of the Related Art
Currently, petroleum is regarded as the world's major energy source. However, as petroleum is a finite resource which is rapidly being depleted, most experts in the industry expect an energy crisis within the next 50 years. High oil prices will greatly impact industries which rely upon petroleum based energy such as industrial electricity production, domestic electricity production, vehicle power, consumer electronics, and mobile communication devices. National and international groups have been searching for alternative energy sources and hydrogen is regarded as one of the more interesting alternatives. Low-temperature fuel cells operated at temperatures of lower than 100° C. include proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). As 3C products have taken on increasingly powerful functions, the requirements of portable energy supplies have become increasingly high. For example, most consumers now insist that their 3C products should be lightweighted, have high energy density, last a long time and be convenient to use. Therefore, low-temperature fuel cells have drawn great attention as substitutes for lithium batteries.
PEMFCs utilize an environmentally friendly electrochemical reaction of hydrogen and oxygen. However, the generation, storage, and transport of hydrogen lead to a big issue to resolve for PEMFCs. In recent years, the development of DMFCs along with PEMs has reached a revolutionary breakthrough in the field of small-power technology. DMFCs have smaller power densities compared to hydrogen-fed PEMFCs. So far it is known that DMFCs have an optimal power density that is only one tenth of hydrogen-fed PEMFCs. Since DMFCs have low power densities, they are suitable for applications in compact portable electronic products, such as laptops, personal digital assistants, and mobile phones. A typical DMFC membrane electrode assembly (MEA) includes a proton exchange membrane (PEM), an electrode catalyst layer, and an electrically conductive layer. DMFCs can convert chemical energy from the liquid methanol fuel into electrical energy. PEMFCs use hydrogen as a fuel source. Compared to PEMFCs, DMFCs do not need additional reformers to convert the fuel into hydrogen. Therefore, the potentially complex assembly of a DMFC can be simplified and thereby its convenience increased. Furthermore, diluted methanol can be used as a by-functional fuel without the need of an additional membrane wetting mechanism.
Currently the biggest bottleneck for the commercialization of DMFCs is their excessively low energy conversion rate. Therefore, most research has been aimed at developing a high-activity catalyst electrode.
DMFCs have a theoretical voltage of 1.18V at 298 K. This voltage value can be obtained from half reactions at the cathode and the anode.Anode:CH3OH+H2O→CO2+6H++6e−Eoanode=0.05 VSHE Cathode:3/2O2+6H++6e−→3H2OEocathode=1.23 VSHE Total reaction:CH3OH+H2O+3/2O2→CO2+3H2OEocell=1.18 VSHE 
The electrochemical reactions for the above cathode and anode usually need the catalysts to reduce the energy barrier for the reaction so as to speed up the oxidization reaction at the anode and the reduction reaction at the cathode. Among various precious metal catalysts, platinum has optimal activity for the oxidization of methanol fuel at the anode and the reduction of oxygen at the cathode. Therefore, most current research is made based on the condition of using Pt as an electrode catalyst. Detailed reactions are listed below:Pt+CH3OH→Pt—COad+4H++4e−  (a)H2O+Pt→Pt—OH+H++e−  (b)Ru+H2O→Ru—OH+H++e−  (c)Pt—CO+Ru—OH→Pt+Ru+CO2+H++e−  (d)Pt—COad+Pt—OHad→CO2+H++e−  (e)Pt—CHOad+Ru—OHad→CO2+2H++2e−  (f)
Methanol absorbs onto the Pt surface to generate CO through a series of de-proton steps (reaction (a)). CO molecules tend to strongly adsorbed on the entire Pt surface to reduce the amount of active sites for catalysis, leading to cell power deterioration. This phenomenon is well known as CO poisoning. If platinum-ruthenium (Pt—Ru) is used for catalysis, a Pt—Ru catalyst in alloy form can effectively reduce CO poisoning. First, the Ru reacts with water molecules to form an Ru—OH (reaction (c)). This Ru—OH compound then triggers a neighboring Pt—COad to induce CO oxidation and form a carbon dioxide molecule (reaction (d)). If Pt—CHOad is formed, a similar reaction can proceed as well (reaction (f)). There is a need to develop a platinum-based catalyst which increases the reaction efficiency of methanol oxidation at the anode of a DMFC.
Furthermore, the electrons generated from methanol oxidation at the anode flow to the cathode through an external loop to provide electric power. Simultaneously, the protons transported through the PEM reacts with oxygen at the cathode to form water (in most of the cases, platinum is used as catalyst). The reasons why the catalyst at the cathode has poor electrochemical activity may be due to the reactions as follows.O2+4H++4e−=H2O Eo298° K=+1.23 VSHE  (g)O2+2H++2e−=H2O2Eo298° K=+0.68 VSHE  (h)Pt+H2O═Pt—O+2H++2e−Eo298° K=+0.88 VSHE  (i)
In the electrochemical reaction at the DMFC cathode, some of the oxygen atoms will be reduced to hydrogen peroxide (reaction (h)) during the reduction of oxygen into water. The surface of the platinum will be oxidized at a higher potential (reaction (i)) so that the potential loss during the reduction reaction at the DMFC cathode is higher than 0.3V. Furthermore, many approaches, such as increasing the thickness of the PEMs or adding a carbon powder layer between the PEMs and the catalyst layer, have been proposed to overcome the problems of methanol crossover. However, those approaches tend to increase interfacial resistance for DMFCs and accordingly degrade the performance of the cell.
In general, the well-distributed and small-sized catalyst contributes to an increase in activity of the DMFC catalyst. There are two commonly-used approaches: one is to use nano-sized carbon materials as catalyst supports to enhance the dispersion of the catalysts, and the other is to change the structure control the alloy composition of the catalysts. For example, a platinum-based dual-alloy or a triple-alloy can be used as an effective catalyst. Furthermore, a nano-sized catalyst usually retains a high specific surface area and easily leads to a full utilization of the catalyst. Therefore, there is a need for nano-platinum based alloy catalysts which would increase the reaction efficiencies of methanol oxidation and oxygen reduction reactions.
Processes which are commonly used to prepare a catalyst electrode of a low-temperature fuel cell include chemical reduction and electrodeposition. In the chemical reduction process, a carbon support is basically immersed in a precursor-contained (such as Pt, Ru, W, Co, Fe, or Ni) solution for several hours. After drying processes, the carbon supported metal or alloyed catalysts are put into a furnace under argon or hydrogen at about 250-300° C. for few hours. Alternatively, hydrogen as reduction agent can be introduced into the aqueous solution for several hours. Platinum or platinum-based alloy nano-particles are deposited on the surface of the carbon supports. Basically, chemical reduction is performed at a well-controlled pH value so that the redox reaction occurs efficiently. Furthermore, the temperature of the chemical redox reaction is within the range of 60° C.-150° C. The chemical reduction for depositing a single metal such as platinum is a well-developed technique; however, adding a neutralizer, such as sodium hydroxide, for controlling the pH value is still necessary. Moreover, the time-consuming chemical reduction allows Na ions to deposit on the carbon supports, resulting in unnecessary contaminations.
When using the electrodeposition process, the particles of a single metal or multiple metals are reduced from a metal precursors (usually chlorides) contained electrolyte with acids such as sulfuric acid, nitric acid, perchloric acid, or hydrochloric acid. A potential, usually a negative potential, is applied on a conductive substrate, so that the substrate becomes negative charged (as a cathode), and a counter electrode (usually a non-polarized electrode such as a platinum electrode) becomes positive charged (as anode). Metallic ions in the solution exchange electrons with the negative substrate and are then deposited onto the substrate. However, the size of the metallic particles prepared by the most commonly used electrodeposition process at present is usually more than 20 nm, resulting in a great decrease in the specific surface area of the catalysts.